A
In the 1960s，while studying the volcanic history of Yellowstone National Park，Bob Christiansen became puzzled about something that，oddly，had not troubled anyone before：he couldn't find the park's volcano. It had been known for a long time that Yellowstone was volcanic in nature — that's what accounted for all its hot springs and other steamy features. But Christiansen couldn't find the Yellowstone volcano anywhere.
Most of us，when we talk about volcanoes，think of the classic cone（圆锥体）shapes of a Fuji or Kilimanjaro，which are created when erupting magma（岩浆）piles up. These can form remarkably quickly. In 1943，a Mexican farmer was surprised to see smoke rising from a small part of his land. In one week he was the confused owner of a cone five hundred feet high. Within two years it had topped out at almost fourteen hundred feet and was more than half a mile across. Altogether there are some ten thousand of these volcanoes on Earth，all but a few hundred of them extinct. There is，however，a second less known type of volcano that doesn't involve mountain building. These are volcanoes so explosive that they burst open in a single big crack，leaving behind a vast hole，the caldera. Yellowstone obviously was of this second type，but Christiansen couldn't find the caldera anywhere.
Just at this time NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone. A thoughtful official passed on some of the copies to the park authorities on the assumption that they might make a nice blow-up for one of the visitors' centers. As soon as Christiansen saw the photos，he realized why he had failed to spot the caldera; almost the whole park-2.2 million acres—was caldera. The explosion had left a hole more than forty miles across—much too huge to be seen from anywhere at ground level. At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans.
21.What puzzled Christiansen when he was studying Yellowstone？
A. Its complicated geographical features.
B. Its ever-lasting influence on tourism.
C. The mysterious history of the park.
D. The exact location of the volcano.
22. What does the second-paragraph mainly talk about？
A. The shapes of volcanoes.
B. The impacts of volcanoes.
C. The activities of volcanoes.
D. The heights of volcanoes.
23. What does the underlined word "blow-up" in the last paragraph most probably mean？
A. Hot-air balloon. B. Digital camera.
C. Big photograph. D. Bird's view.
